Nowhere are Saudi Arabia's worries about its fragile security and precarious oil-based future more apparent than in mountinious North Yemen.

A formal Saudi ally-critics even say satellite North Yemen is seeking Western, especially American, help to defend itself not only from military threats from Marxist South Yemen, but also from encroachments by its oil-rich neighbor and nominal protector.

Much of the Saudis' dilemma here is of their own making. Unless they are both astute and lucky, their past errors may already have set in train events that could turn strategically located North Yemen into the Arabian Peninsula's equivalent of the running sore that has all but destroyed Lebanon on the Miditerranean.

A possible indication of things to come was provided last night when Syria, Libya and the Palestine Liberation Organization walked out of an Arab ministers' meeting in Baghdad to protest Saudi Arabia's refusal to carry out stiff sanctions against Egypt for signing a peace treaty with Israel. Iraq, as host to the conference, did not walk out, but backed the three hard-liners in their demands for tough reprisals.

At the same time in Kuwait, Iraq and Syria were deeply involved in Arab League efforts to solidify the nominal cease-fire between North and South Yemen, designed to prevent further fighting almost all observers fear is inevitable. Not much imagination is required to summon forth a scenario in which Syria and Iraq could show their displeasure at the Saudis by ceasing to press their military Marxist friends in South Yemen to behave themselves. Given the Aden government's military superiority - which $400 million worth of saudi-financed American hardware is not expected to dent significantly for some time - another South Yemen push would then seem a distinct possibility.

As was the case in last month's border war, South Yemen's hundreds of Soviet and Cuban military advisers could be counted on to further the Kremilin's long-term objective of softening up Saudi Arabia.

The Saudia predicament became patently clear last month during the Yemeni border fighting when the Riyadh government did nothing more than cancel leave for its 60,000-man armed forces and withdraw its peacekeeping troops from Lebanon. Such relative impotence was the logical result of longstanding Saudi indecision over Yemeni policy.

Had the Saudis actively implemented a 1975 agreement to provide North Yemen with modern arms to meet the South Yemeni military threat Aden might well have thought twice before launching the border war. Instead, the Saudis dragged their feet.

They were certainly concerned about South Yemen's Marxism, which makes no secret of its hopes of overthrowing the Saudi royal family as well as other conservative Arab governments. But they were equally fearful of turning the buffer in Sanaa into anything smacking of a major military power.

With its 6 million inhabitants, North Yemen is more heavily populated than the rest of the Arabian Peninsula combined.

Nor are North Yemenis entirely happy with Saudi Arabia, despite rich official praise. Saudi Arabia provides $200 million in annual grants, and work for more than a million Yemenis who remit an estimated $1.5 billion a year.But, as one Yemeni official recalled, "The Saudis are our natural enemies."

Saudi Arabia annexed two Yemeni provinces in 1934. Today the Yemenis, who like to recall they are descendants of the queen of Sheba and look down their noses at the latecomers in Riyadh, resent the treatment afforded their hardworking kinsmen in Saudi Arabia.

Saudi arms and money helped keep alive the nasty civil war from 1962 to 1969 that pitted their royalist proteges against the republicans who finally won. Saudi influence nevertheless has been widespread and often resented.

The series of political assassinations that appear to be the favored Yemeni recipe for transferring power are often traced to the Saudis.

Col. Salh Hudayan, the Saudi military attache, by local reputation is able to involve the still powerful northern tribes in just enough trouble to dissuade any government in Sanaa from decisions judged inimical to Saudi interests.

Frightened by the implications of the "arc of crisis"-the new diplomatic codeword for trouble extending from the Horn of Africa to Pakistan-the Saudis may well decide to back North Yemen with fewer reservations than in the past.

But Saudi alternatives appear limited. "The next time it comes to finding a new North Yemeni prresident," a diplomat said, "it won't just be the Saudis making the decisions. Iraq, for one, is very much around."